As is known, the speech circuit of an electronic telephone apparatus is supplied with electrical energy provided on the user's line from the so-called central battery. A suitable circuit is arranged to extract the necessary electrical energy from the line, to provide it at a stabilized voltage to the speech circuit to be supplied.
A known circuit of this type, which is shown in the block diagram of FIG. 1 of the drawing accompanying the present specification, comprises a capacitor C which is connected on one side to one wire of the line loop, indicated with the earth symbol, and, on the other, to the other wire L of line loop, via the series connection of a resistor R2, the collector-emitter path of a transistor T and a diode D connected in a forward biased sense. In reality, as is known, both the earth terminal and the wire L are connected to the wires of the line loop by other components, not shown in order not to excessively complicate the drawing. The transistor T is held in conduction in the saturation region by the output of an operational amplifier A the inverting input terminal of which is connected to the connection node between the resistor R2 and the emitter of the transistor T, and the non-inverting input of which is also connected to the line L via a second resistor R1. The connection between the emitter of transistor T and the inverting input of the operational amplifier A constitutes a negative feedback loop which tends to maintain the voltage drops on the resistors R1 and R2 and therefore the voltages on the input terminals of the amplifier equal to one another. Between the non-inverting input of the amplifier and earth there is shown a current generator G to indicate that a control current Ic, proportional to the current flowing in the line and derived from known means, not shown, is applied to the non-inverting input of the amplifier A. A regulation circuit SR, connecting between the collector of the transistor T and earth, acts to stabilize the voltage VS across the capacitor C. The voltage Vs is applied to the speech circuit, not shown, in order to constitute its supply.
The maximum obtainable value for the stabilized voltage obviously depends primarily on the value of the voltage on the line at the connection point with the user's apparatus, which is given by the voltage of the central battery reduced by the voltage drop on the line due to the resistance of the line itself and by the so-called "voltage loss" that is to say the voltage drop along the path between this connection point and the effective stabilized voltage take-off point.
In the design of telephone networks it is necessary to arrange that the user's line does not reach a length such that the voltage at the connection points to the user's telephone apparatus is less than a minimum predetermined value which still allows the electrical energy to be taken from the line at a sufficiently high voltage to supply the speech circuit.
The stabilized supply voltage VS is given by the DC voltage VL of the line at the connection point of the user's apparatus reduced by the "voltage loss". For the above described known circuit the "voltage loss" is equal to the sum of the voltage drop VR1 on the resistor R2, the collector-emitter voltage V.sub.CEsat of the transistor T in saturation and the voltage VD of the forward biased diode D.
The stabilized voltage is, therefore: EQU VS=VL-(VR2+V.sub.CEsat +VD)
Since, typically, V.sub.CEsat and VD are both about 0.3 V and, at the usually operating currents, VR2 for a resistor of 10 ohms, is about 0.1 V the overall voltage drop is about 0.7 V. Now, the stabilized voltage necessary for supplying a normal speech circuit is about 3 V, and therefore the voltage at the connection point of the resistors R1 and R2 to the line cannot be less than 3.7 V. It is obvious that this puts a limit on the maximum length of the user's line.